Biometric fingerprint readers are currently used in many locations as a means of identifying particular individuals. In general, these devices operate to scan a person's fingerprint into an electronic format. Once this electronic copy of the fingerprint has been obtained, this fingerprint may then be compared with a database to determine if the person that gave the fingerprint is a criminal, terrorist, or other individual wanted by a law enforcement agency. It is for this reason that fingerprint readers are often used at border checkpoints, airports, and other public locations as a means of detecting known criminals.
Typical examples of fingerprint scanners disclosed in the art are found in U.S. Patent Application Publication No. 2004/0252867 and U.S. Pat. No. 5,892,599. Both of these documents are expressly incorporated herein by reference.
Further, fingerprint scanners are also used as a security feature on buildings, briefcases, safes, and/or other secure locations. Specifically, the fingerprint scanner obtains the electronic copy of the person's fingerprint and then compares this fingerprint with a known database to determine whether this individual is authorized to enter the building, open the safe, etc. If the person's fingerprint matches one of the records in the database, the building, briefcase, secure area, etc. will immediately be unlocked and the person will be granted access to the secure location. Of course, if the fingerprint does not match with known records, access to this secure location will be denied.
The above-recited list is but two examples of current usages for fingerprint readers. Other potential applications and usages for fingerprint readers are also possible.
Some fingerprint readers operate using the principle of “total internal reflection” or “TIR”. More specifically, the fingerprint reader contains an optical window designed such that when light is shined on the internal side of this optical window, the glass will totally reflect this light internally. However, when a person places his or her finger on the optical window (and light is added), the natural oils found in the person's finger frustrates/overcomes the TIR properties of the glass. Accordingly, some of the light will pass through the optical window and some of the light will be reflected back into the reader. More importantly however, the light that is reflected back into the reader contains an image of the person's fingerprint. Thus, if this reflected light is directed onto a camera (or other recording device), and electronic image of the person's fingerprint may be obtained. Once this electronic image of the fingerprint is obtained, this image may then be processed, compared to a database, or otherwise manipulated as desired.
Unfortunately, with fingerprint readers that use TIR, there is a problem with ambient light that must be addressed. Specifically, it is possible for ambient light (from the outside of the reader) to pass through the optical window where the finger is placed. If this ambient light reaches the camera, it will degrade the image of the fingerprint and/or reduce the precision/sensitivity of the fingerprint image.
Ambient light can also saturate the camera and can make detection of the light containing the fingerprint image very difficult. In fact, ambient light reaching the camera may be ten to twenty times brighter than the light containing the fingerprint image. Given that the camera will generally use a “wide-open” camera setting to gather as much light as possible, this ambient light can simply “drown out” the light containing the image. When this occurs, the ability of the reader to produce a precise fingerprint image is greatly diminished.
Further, when ambient light is allowed to reach the camera, a latent fingerprint that is left on the optical window (i.e., a fingerprint from a previous person) can be detected. The latent image can also be intentionally illuminated with an outside light source and fool the system.
In order to compensate for the effects of ambient light, a filter is often used in conjunction with the camera. More than one filter may be used. These filters are designed to select a narrow spectrum of light reaching the camera, thereby reducing any stray images that may be present in the ambient light. The use of these filters reduces the amount of ambient light that can enter the camera. However, even with the use of a filter, there is still the narrow spectrum of light (that is present in ambient light or intentionally introduced) that can pass though the filter and can still reach the camera. Thus, even with the use of filters, many fingerprint readers still must be surrounded by a dark area (i.e., free of ambient light) in order to achieve maximum performance and sensitivity.
Accordingly, a new type of filter is needed. Such a device is disclosed herein.